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Technical Bulletin TB-P5
The Superiority of the Differential pH Electrode System
Conventional pH Measurement Methodology
All pH measurement systems operate on the principle
of an electrochemical cell; that is, when two dissimilar
conductors are placed in an electrolyte (a liquid
capable of carrying a charge) they will develop a
potential between them. The magnitude of this
potential is dependent on the characteristics of the
conductors and the electrolyte. The measurement of
pH depends on a phenomenon associated with a
particular kind of glass which responds to the hydrogen
ions that are present. A conventional pH sensor is
basically a high impedance battery. The high
impedance results from one of the battery plates being
"glass coated." A conventional pH measurement
system, shown in Figure 1, is comprised of a pH glass
electrode and a pH reference electrode.
The glass electrode, common to all systems, is
designated E1. The other half cell, the reference
electrode, is designated E2. The reference electrode is
a closed cylinder, usually made of plastic that contains
a silver wire coated with AgCI (silver chloride), and a
filling solution, gel, or crystals of saturated potassium
chloride (KCl). The electrical circuit to the process
being measured is completed through a porous plug
junction in the tip of the electrode. The junction may be
ceramic, wood, Teflon, or a small hole. These liquid
junctions provide an electrical path but prevent the
reference electrode fill solution from flowing freely into
the process. The output of the cell is:
The pH and pC1 components are the logarithms of
hydrogen and chloride ion concentration, respectively.
If pC1 is constant, then:
pH = K (E1 – E2)
Therefore, the output of the cell is proportional to a
constant (K) times the potential of the glass electrode
(E1), which is a function of the hydrogen ion
concentration, less the potential of the reference
electrode (E2), which is a function of its chloride ion
concentration. Since pH measurement is the negative
logarithm of hydrogen ion concentration, it becomes
apparent that the accuracy of such a cell depends on
the constancy of the potential of the reference
electrode.
Figure 1
Conventional pH Measurement System
pH
Glass
Electrode
pH
Reference
Electrode
(pH – pC1) = E1 – E2
Conventional pH System Weaknesses
A conventional pH measurement system has
several weaknesses. One shortcoming is due to the
dependence of unbuffered chloride ion concentration in
the reference electrode. Any contamination of this KCl
fill solution will cause error. Consider the case of the
reference electrode in clear water. Even in as mild an
environment as water, chloride ions will migrate out of
the reference electrode and water will migrate into the
electrode, diluting the reference electrode fill solution.
tech bulletin TB-P5.f
A dilution of 100 to 1 can cause a possible
measurement shift of 2 full pH units! This infers that
long before the reference electrode solution becomes
diluted, the error becomes unacceptable. When an
unbuffered solution is used in a conventional reference
electrode, any dilution, no matter how slight, represents
some error, and relatively minor dilutions represent
substantial error.
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The Superiority of the Differential pH Electrode System
Technical Bulletin TB-P5
The second shortcoming of a conventional pH
measurement system is that, since chloride ion
concentration cannot be kept constant, it is a practical
necessity to provide the means to occasionally
replenish the KCl fill solution. Such replenishment
requires opening the sensor to add a gel, crystals, or
a solution, or to replace a throwaway style reference
electrode.
A pH sensor, which is always immersed in the process,
contains electronics that must be kept dry, and requires
a means for maintenance access. These requirements
make the sensor very vulnerable to leakage.
Mechanically, the sensor usually has machined
surfaces and O-rings or other gasketed seals. Such
construction is relatively expensive and does not
positively preclude the possibility of leaks. When
process fluid leaks into a sensor, it generally destroys
the entire sensor which then must be replaced.
Understanding the third major weakness of a
conventional pH measurement system requires an
overview of the total system (sensor and analyzer).
The sensor, normally used in a vessel or pipe, makes
electrical contact through the vessel or pipe and its
contents to an earth ground. The difference in the earth
potential to which the vessel or pipe is connected and
the earth potential to which the analyzer is connected
can be represented as a ground loop voltage source.
By Ohm's Law, most of that ground loop current will
flow through the lowest impedance path of the sensor.
Since the glass electrode has extremely high
impedance, essentially all of the ground loop current
flows through the reference electrode.
In practical applications, it is not uncommon to find that
the chemical makeup of the solution being measured is
such that it leaves some precipitate on the sensor
elements. Any precipitate coating on the reference
electrode creates a resistance in the path of the ground
loop current. This is the fourth weakness of a
conventional system. According to Ohm's Law, there
will be a voltage drop across this resistance. The
measurement system cannot distinguish this voltage
change from a voltage change caused by a change in
pH. Consequently, the voltage drop due to ground loop
current causes a measurement error that is
proportional to the magnitude of the ground loop
current and the added resistance of the reference
electrode coating.
Summary of Weaknesses
• Potassium chloride fill solution used in the
reference electrode cannot be buffered. Thus, any
contamination or dilution of this solution will result
in relatively large measurement errors.
• Changes in the fill solution necessitate dismantling
the sensor to replace the reference electrode. The
result is more downtime and greater risk of glass
breakage. Additionally, the need to access the
reference electrode makes the sensor's
mechanical design inherently more complex,
resulting in higher initial cost and greater
vulnerability to leakage.
• All ground loop current flows through the reference
electrode, causing measurement error that is
proportional to the magnitude of the current.
• Precipitate deposited from the process solution
onto the reference electrode causes errors in
measurement, due to ground loop voltage drop
across the resistance of the precipitate.
Hach's Differential Electrode Technique, using our fieldproven Standard Electrode, overcomes all of these
shortcomings.
The Differential Electrode Technique
Rather than a reference electrode in KCl solution, the
Differential Electrode Technique, patented by Great
Lakes Instruments (GLI International, Inc., a Hach
Company brand), uses two glass electrodes to make
the measurement differentially with respect to a third
metal electrode. The Standard Electrode consists of a
glass electrode in a chamber which is filled with a
concentrated, buffered solution of pH 7. This inner
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The Superiority of the Differential pH Electrode System
chamber makes electrical contact with the process
through a double junction salt bridge. The second glass
electrode is exposed to the process.
The Standard Electrode assembly is shown as E2 in
Figure 2. The glass electrode in this assembly is kept in
a constant pH environment. As the figure shows, the
electrode is contained within a chamber filled with a
Technical Bulletin TB-P5
7 buffer (standard cell solution). The inner chamber
makes electrochemical contact with a salt bridge
chamber through a ceramic plug. The use of a second
glass electrode to create a standard cell permits the
use of a buffer solution which, by definition, resists pH
change.
A unity gain amplifier amplifies the output of each
electrode to reduce the impedance of the system. The
output of the first unity gain amplifier is the voltage of
the process electrode (E1) minus the voltage of the
metal, solution ground electrode (E3). The output of the
second unity gain amplifier is the voltage of the Standard
Electrode (E2) minus the voltage of the metal, solution
ground electrode. The circuitry of the analyzer
subtracts the outputs of the unity gain amplifiers,
canceling the E3 term.
pH = K [(E1 - E3) - (E2 - E3)] = K [E1 - E2]
Figure 2
The Hach Differential Electrode
Technique
Preamplifier
Temperature
Compensator
Process
Electrode
(E1)
Reference
Electrode
(E2)
Solution
Ground
Electrode
(E3)
Benefits of Differential Electrode Technique
The Hach Differential Electrode Technique system
does not depend on a reference electrode or KCl
solution. The Hach Standard Electrode maintains its
pH level regardless of substantial dilution. A dilution of
the Hach Standard Electrode to 100 to 1 represents
only a 0.05 pH shift! This is a very favorable
comparison with the shift of 2 full pH units in the
conventional pH measurement system example
previously described.
The associated chambers contained in the Standard
Electrode are sealed, and the only entrance from
chamber to chamber is through the tight ceramic
junctions which provide electrical contact to the
solution being measured, thereby completing the
circuit. Any contaminant that could find its way into the
inner chamber of this assembly must first enter through
the outer junction. It is apparent from this configuration
that it is extremely unlikely that any appreciable
amounts of contaminant could find its way to the inner
chamber. If this were to occur, however, the inner
chamber is filled with a buffer that could receive a
dilution upwards of 100 to 1, with essentially no effect
on the pH environment of the Standard Electrode.
Since the Hach Standard Electrode is comprised of a
sealed glass measuring electrode, the silver/silver
chloride wire cannot be contaminated. Also, the
standard cell solution contained in the Standard
Electrode usually lasts for a year or more in typical
process and wastewater applications. The salt bridge
and standard cell solution can be replaced periodically
at a very low cost, making the Hach Differential sensor
economical to maintain. Furthermore, the Hach sensor
is constructed with a specially designed housing into
which all of its components are inserted. Then the
assembly is completely encapsulated. The combination
of all of these features provides a vastly superior pH
sensor, available at the lowest cost of ownership, with
not only higher performance, but also unparalleled
reliability due to its simplified construction.
Tests confirm that, due to the inherent stability of the
Standard Electrode, the Hach Differential sensor is
very stable over a wide spectrum of typical
applications.
The Hach Differential Electrode Technique also
provides a unique way to bypass ground loop currents
that would otherwise cause measurement error. The
Hach Differential sensor uses two glass electrodes to
make the measurement differentially with respect to a
third metal ground electrode, purposely eliminating the
conventional reference electrode.
One of the shortcomings of conventional systems
described earlier is the effect of ground loop currents.
In a measurement system using a Hach sensor, ground
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The Superiority of the Differential pH Electrode System
Technical Bulletin TB-P5
loop currents flow through the path of least resistance,
which is through the low impedance metal ground
electrode. Any precipitate build-up on this electrode
can be the source of a voltage drop. However, this
voltage simply modifies E3, but as the earlier equation
shows, E3 is mathematically cancelled, and does not
cause an error in the pH measurement.
Furthermore, any resistance that occurs due to
precipitate forming on the salt bridge of a Hach sensor
is in series with a high impedance glass electrode. The
effect of such precipitate resistance is minimal. On a
conventional reference electrode, any precipitate
resistance will affect the measurement because it is in
series with a low impedance electrode. For these
reasons, the Hach Differential sensor will provide
measurements of undiminished accuracy over far
longer periods of time than a conventional sensor.
By its defined function, the unity gain amplifier
associated with each electrode (Figure 2) provides a
millivolt output that is equal to the millivolt input. Its sole
purpose is to reduce the impedance of the system. The
50 megohms impedance of a glass electrode appears
as one-tenth ohm at the output of the unity gain
amplifiers. These amplifiers are very reliable, enabling
them to be imbedded in the sensor to provide a pH
measurement system that acts as a relatively low
impedance source. This allows for a separation
distance between the analyzer and sensor of up to
3000 feet.
Summary of Benefits
The patented Hach Differential Electrode Technique is
better by design:
Double-junction salt bridge of the Hach Standard
Electrode makes it extremely unlikely that an
appreciable amount of contaminant can find its way to
the inner chamber. In addition, the nature of the
Standard Electrode buffer is such that a 100 to 1
dilution would represent a change in the measurement
of only 0.05 pH. With a conventional system, a similar
level of dilution could cause a shift of two full pH units.
The Hach Differential sensor is a simple and
economical encapsulated sensor with a removable salt
bridge to provide access to the Standard Electrode
buffer while sealing the sensor preamplifier from
moisture.
The Hach Differential pH Sensor provides a definitive
solution to the problems common with conventional
systems: errors due to reference contamination, and
errors due to precipitate build-up on the sensor. Also,
the Hach Differential sensor provides a unique solution
to the third shortcoming of conventional systems. That
is, the need for a sensor which can be easily
dismantled.
All Hach Differential pH sensors are manufactured so
that the double junction salt bridge and the Standard
Electrode buffer may be removed and replaced in
the field.
Compared to conventional techniques, the
patented Differential Electrode Technique provides
measurements of greater stability over longer periods
of time with less downtime and maintenance.
Since ground loop currents do not pass through the
Standard Electrode in the Hach Differential sensor, the
reference signal is not affected by precipitate buildup
on the reference components of the sensor.
FOR TECHNICAL ASSISTANCE, PRICE INFORMATION AND ORDERING:
In the U.S.A. – Call toll-free 800-227-4224
Outside the U.S.A. – Contact the Hach office or distributor serving you.
On the Worldwide Web – www.hach.com; E-mail – [email protected]
© Hach Company, 2005. All rights reserved. Printed in the U.S.A.
HACH COMPANY
WORLD HEADQUARTERS
Telephone: (970) 669-3050
FAX: (970) 669-2932
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